1. Field of the Invention
The present invention relates to non-destructive methods and apparatus for determining the structural integrity of materials. More particularly, the present invention relates to the combination of laser vibrometry and vibrational analysis techniques for determining macro- and microstructural irregularities in materials. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-898R18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Material testing for quality control and related areas continues to be mostly destructive in nature despite efforts to develop non-destructive alternatives that are more feasible in terms of price, convenience and reliability. Although destructive testing is quite often more accurate because the condition of the material is made manifest rather than inferred, the obvious disadvantage is that the material or product tested is destroyed or rendered useless by the testing process.
Most non-destructive testing evaluates material composition and structure by relying on the interaction of the tested material with sound waves or electromagnetic radiation. Such methods involve monitoring the effect of pressure or electromagnetic waves passing through the material as they are influenced by flaws or inhomogeneities in the test structure.
The analysis in most non-destructive testing of this type relies on the relationship between the material's resonant frequency and the strength and quality of the material's structure. The resonant frequency of a material depends upon, among other things, the material's shape, density, stiffness and the like.
Typically, the tested material structure is vibrated using a known force and the vibrational velocity of the tested area is measured. Using known techniques, the data is used to compute the resonant frequency of the tested area. Qualitative evaluations are then made based on the resonant frequency using known relationships.
Means for non-destructively vibrating test objects include gas jets, magnetic fields, acoustic, optical and electro-mechanical waves, all of which are discussed in U.S. Pat. No. 4,641,527, issued to Hiroi, et al. Alternatively, acoustic waves produced by transducers can provide necessary vibration, as disclosed in U.S. Pat. No. 4,824,250, issued to Newman, and U.S. Pat. No. 4,723,448, issued to Veligdan. Suitable vibrational velocity measuring means include laser vibrometers or other means for comparing the direction and reflection of laser beams about a vibrated area. For example, see U.S. Pat. No. 3,482,436, issued to Neish, et al. for a vibration-responsive apparatus.
Laser beams are known for use in non-destructive testing to detect structural defects. For example, in U.S. Pat. No. 3,604,253, a laser beam is projected onto a test object, the object is vibrated and the pattern of light reflected from the object is analyzed. As the frequency and intensity of vibrations are varied, changes appear in the pattern of light, thus indicating defects in the object. Also, J. W. Lemmens, Inc. has developed a non-destructive materials testing system that makes use of the relationship between resonant frequency and the structural soundness of materials.
A less common characteristic feature used in non-destructive testing of material structures is the phenomenon of damping. Damping, in general, refers to a material's ability to absorb externally-induced vibration due to its microstructure. Measurements of damping characteristics, such as damping factors and damping loss factors, are typically used in vibration control applications. Damping loss factors have not been used in determining or evaluating the strength, degree of abnormalities or overall quality in material structures and bonded areas between structures, such as solder joints, welds and the like.
Vibrational analysis methods for measuring damping characteristics of materials, such as the resonance dwell method, are well known for vibrational control applications. In the resonance dwell method, the damping loss factor is determined from the width of the mobility curve as a function of frequency at the half power points. In short, the width of the mobility curve at this point divided by the resonant frequency around which the mobility curve is plotted, yields the damping loss factor of the inspected area.
There is a need for a more effective, non-destructive testing method for determining the integrity of material structures and bonding means therebetween.